1. Field of the Invention
The present invention relates to a method for forming self-assembled colloidal photonic crystals on a selected area of a substrate or forming self-assembled colloidal photonic crystals having different diameters on the same substrate using an electric field, and a method for fabricating three-dimensional photonic crystal waveguides of an inverted-opal structure using the self-assembled colloidal photonic crystals patterned by the patterning method.
2. Description of the Related Art
Photonic band gap structures in photonic crystals composed of dielectrics having a three-dimensional periodicity have become a matter of increasing interest. The photonic band gap structures are highly applicable to diverse optoelectronic devices such as microlasers, filters, high-efficiency LEDs, optical switches, low-loss waveguides, etc. In an initial stage of investigation, a three-dimensional photonic band gap in a microwave range was realized by providing periodicity to the dielectric constant of a dielectric-air structure by making small holes in a parallel direction on a silicon wafer or by stacking bar-shaped dielectrics in piles. However, in the wavelength ranges of infrared rays and visible light, it was only possible to form a two-dimensional photonic band gap. This is because it was very difficult to form a three-dimensional photonic band gap due to the need for scaling-down the etching space. In the case of a three-dimensional photonic band gap, the holographic lithography of laser light was used. Recently, research for methods of self-assembling small spheres (colloids) having a diameter of several hundred nanometers (nm) has been conducted.
In particular, diverse methods for fabricating self-assembled colloidal photonic crystals have been studied. One of the methods most frequently used is a dip-coating method for fabricating photonic crystals which utilizes capillary force exerted among a colloidal fluid, a substrate and colloidal particles. This dip-coating method is easy to carry out, and can form photonic crystals having a high crystallization in a wide area. However, it is difficult to selectively control the colloidal particles, and many semiconductor processes such as the lithography are required for patterning the photonic crystals. Additionally, in the case of forming photonic crystal of different kinds of colloidal particles or colloidal particles having different sizes, a template is required. Also, in the case of forming colloidal photonic crystals using three or more kinds of colloidal particles or colloidal particles having three or more sizes, the fabricating processes become complicated with limitations in design.
The most frequently used method for fabricating waveguides using photonic crystals comprises forming a transmission line from two-dimensional photonic crystals formed by periodically etching fine holes onto a silicon substrate. The two-dimensional photonic crystal has a photonic band gap and therefore results in no photonic loss in a two-dimensional light-traveling direction, but does suffer photonic loss in other light-traveling directions.
Meanwhile, because the three-dimensional photonic crystal of an inverted-opal structure has a photonic band gap in all three-dimensional light-traveling directions, a waveguide using these three-dimensional photonic crystals can greatly reduce photonic loss in comparison to a two-dimensional photonic crystal. The fabrication of a waveguide having the three-dimensional photonic band gap requires a high-grade etching technique such as e-beam lithography, a high fabricating cost and considerable manufacturing time, and it is difficult to implement the waveguide in a wide area. Accordingly, there has been an increasing demand for and interest in a method for fabricating three-dimensional photonic crystal waveguides of an inverted-opal structure through self-assembly of colloidal particles that can easily allow for fabricating photonic crystals in a wide area.
Conventional methods for fabricating three-dimensional photonic crystal waveguides of an inverted-opal structure have been proposed as follows.
First, the method described in “Multi-Photon Polymerization of Waveguide Structures within Three-Dimensional Photonic Crystals”, Advanced Materials, vol. 14, 2003, pp 271–294 by W. Lee et al. has problems due to material and area limitations. Second, the method described in “Micromolding of Three-Dimensional Photonic Crystals on Silicon Substrates”, Nanotechnology, vol. 14, 2003, pp 323–326 by P. Ferrand et al. also has problems in that its process is complicated and the photonic crystals are broken with the occurrence of cracks during an artificial piling up of the photonic crystals.